Systems and Methods Related to Identifying and/or Locating Weapon Fire Incidents

ABSTRACT

A system and method for detecting, identifying, and fixing the location of the source of an acoustic event. The inventive system includes: a plurality of sensors dispersed at somewhat regular intervals throughout a monitored area; a communication network adapted to deliver information from the sensors to a host processor; and a process within the host processor for determining, from the absolute times of arrival of an event at two or more sensors, a position of the source of the event. Acoustic events are detected and analyzed at each sensor so that the sensor transmits over the network: an identifier for the sensor; an identifier for the type of event; and a precise absolute time of arrival of the event at the sensor. In a preferred embodiment, the system also identifies the type of weapon firing a gunshot.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/905,788, filed Jan.20, 2005, published as publication No. 2005/0237186A1, which claimspriority to Provisional Application No. 60/481,922, filed on Jan. 20,2004, and is a continuation-in-part of application Ser. No. 10/248,511,filed Jan. 24, 2003, now U.S. Pat. No. 6,847,587, all of which areincorporated herein by reference in entirety.

BACKGROUND

1. Field

The present invention relates to a system and method for identifying andlocating an acoustic event. More particularly, but not by way oflimitation, the present invention relates to a system for identifying anexplosive event, such as a gunshot, at a remote location, reporting theevent including a precise time of arrival to a host system, andcalculating a location of the source of the event in the host system.

2. Description of Related Art

Generally speaking, there is a long felt need for a system and method topinpoint the exact location of the source of gunfire, particularly in anurban setting. In many large cities, gun-related violence has become aplague of epidemic proportions. It is estimated that roughly 10,000Americans die each year from gun related injuries with an estimated200,000 non-fatal injuries. Recent events such as the so-called SuburbanSniper in the Washington D.C. area have further underscored the need toresolve this problem. Unfortunately, prior art solutions have beeneither inadequate or too costly to gain wide acceptance in the field.

In a typical gunshot locating system, a plurality of sensors aresituated in the field, usually at fairly regular intervals along an x-ygrid. Each sensor includes a microphone and, presumably, an amplifier toproduce an audio signal. The audio signal is then carried by a dedicatedtelephone line to a central location where the sound is processed. Upondetecting a gunshot from the processed audio, relative times of arrivalsat the central location are processed to determine a location of thesource of the gunshot.

One such system, U.S. Pat. No. 5,973,998 issued to Showen, et al.discloses a system wherein sensors are placed at a density of roughlysix to ten sensors per square mile. Showen takes advantage of anaturally occurring phenomenon known as spatial filtering to improve theaccuracy and reliability over that of prior systems. While the Showensystem radically reduces the sensor density compared to prior artsystems, as high as 80 sensors per square mile, each sensor,nonetheless, still requires a dedicated phone line. As one can see, toeffectively monitor a large metropolitan area, an outrageous number oftelephone lines would be required, resulting in a substantial investmentin infrastructure, not to mention large on-going costs. Such systems canoften take months to install.

In addition to the large number of dedicated phone lines required byprior art systems, such systems transport audio information overcomparatively long distances. The signals, as such, are subject to anumber of degrading factors such as noise, crosstalk, inadvertentdisconnection, and the like. Such factors may cause gunshots to goundetected. Latency in one or more communication channels will cause thesystem to produce and erroneous location.

Another known method for identifying the location of a gunshot relies ona special sensor having several microphones arranged in a geometricarray. A radial direction can be determined by measuring the differencesin arrival times at the various microphones. Unfortunately, such systemssuffer from limited accuracy in the determination of the radial angle,which in turn, translates into significant errors in the positionalaccuracy of the source of the noise when triangulation of two or moresensors is performed. Since errors in the radial angle result in everincreasing position error as the distance from the sensor to the sourceincreases, the reported position will be especially suspect toward theouter limits of the sensor's range.

Another type of gunshot sensor detects a gunshot and attempts toidentify a particular type of weapon, or at least a class of weapon.These systems generally analyze the duration, envelope, or spectralcontent of a gunshot and compare the results to known samples. Combininga trustworthy identification of a weapon with the precise location of ashot fired by the weapon would be particularly useful in the earlystages of a police investigation and could allow early correlation of acrime to a repeat perpetrator.

It is thus an object of the present invention to provide a system fordetecting an acoustic event, such as a gunshot, identifying the acousticevent, and fixing a location of the source of the event.

It is a further object of the present invention to provide a gunshotdetection system which can be deployed over a large area withoutincurring undue costs in infrastructure and undue recurring costs.

It is still a further object of the present invention to provide asystem for fixing the location of an acoustic event with greateraccuracy than has been possible with existing systems.

It is still a further object of the present invention to provide asystem for fixing the location of an acoustic event that can be rapidlydeployed.

SUMMARY OF INVENTION

The present invention provides a system and method for detecting,identifying, and fixing the location of the source of an acoustic event.In a preferred embodiment, the inventive system includes: a plurality ofsensors dispersed at somewhat regular intervals throughout a monitoredarea; a communication network adapted to deliver information from thesensors to a host processor at a central location; and a process withinthe host processor for determining, from the absolute times of arrivalof an event at two or more sensors, a position of the source of theevent.

In a preferred embodiment, each sensor includes: a microphone forreceiving an acoustic event; a processor for discriminating acousticevents from other sounds; a synchronized clock; and an interface to thecommunication network. The sensor's processor continuously monitorsenvironmental sound in the proximity of the sensor and detects eventsknown to produce sound of a particular class. Upon detecting an event,the type of event and the precise time of arrival of the event aretransmitted to the host processor via the communication network. Sinceeach sensor includes a real-time clock synchronized to the real-timeclocks of the other sensors in the network, latency in the delivery ofinformation to the host computer will not affect the accuracy of theposition calculation. Accordingly, large numbers of sensors can share acommon communication channel, unlike prior art systems which require adedicated channel for each sensor.

In one preferred embodiment, the sensor further includes storedsignatures of various types of firearms. Each detected event is comparedto the table of known firearm types. The best-fit between the detectedevent and one of the stored types is then transmitted to the host systemalong with a quality value indicative of the degree to which thedetected event correlates with the known sample.

In another preferred embodiment, the synchronized clock is a GPSreceiver. The GPS receiver also provides the location of the sensor,which is periodically reported to the host processor. Global positioningsatellite systems (“GPS”) are well known in the art. Such systemstypically consist of a constellation of satellites in earth orbit. Eachsatellite includes a highly accurate clock and periodically transmits,among other things, time codes back towards earth. By processing thedifferences in time delay of the signals received from severalsatellites, it is possible to calculate the position of a receiver inthree dimensions. With regard to the present invention, however, anadditional value of the GPS system lies in the fact that clocks atmultiple sites, virtually anywhere in the world, may be preciselysynchronized, through received GPS time codes. It is generally held thattwo or more clocks may be synchronized within 350 nanoseconds of eachother, independent of their respective locations. As will be appreciatedby those skilled in the art, the distance traveled by an acoustic wavein free air over 350 nanoseconds is negligible.

In still another preferred embodiment, the inventive sensors areintended to hang from existing power lines. Such power lines aretypically elevated which will improve the “view” of the sensor towardsacoustic events and virtually eliminate the risk of tampering. Anotheradvantage lies in the fact that a sensor may also draw power from thepower line through induction, even though no physical electricalconnection is present. Preferably, a toroidal-type coil is placed aroundthe power line such that AC current flowing in the power line willinduce a voltage in the coil sufficient to operate the circuitry of thesensor. In addition, power line communication is possible over somewhatlimited distances by driving the coil with a modulated carrier encodedwith digital information. The carrier rides on the electrical powertransmitted over the power line.

In yet another preferred embodiment, the sensors lying within a givenarea are each configured to transmit information concerning receivedevents over a radio frequency link. Each unit is programmed to transmitinformation in a manner which will ensure collisions do not occur, or,if a collision does occur, retransmissions will not collide.

In yet another preferred embodiment, a battery-powered sensor isconfigured to be worn by a soldier or law enforcement officer. When thesensor detects an event, the time of arrival information andcharacteristics of the event are transmitted to a host processor foranalysis. In a battlefield environment, the information can be used todirect return fire, program guided weapons, or to direct a laser forlaser guided weapons. In any event, the person wearing the sensor can beprovided a handheld computer, such as a PDA, which displays sources ofgunfire relative to the user.

Further objects, features, and advantages of the present invention willbe apparent to those skilled in the art upon examining the accompanyingdrawings and upon reading the following description of the preferredembodiments.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts the general environment in which the inventive system andmethod are used.

FIG. 2 provides a perspective view of a sensor for use in the inventivesystem adapted to inductively couple electrical power from a power line.

FIG. 3 provides a perspective view of the sensor of FIG. 2 with the clipopened for attachment to a power line.

FIG. 4 provides a perspective front elevation view of a sensor for usein the inventive system which may be worn by a soldier or policeofficer.

FIG. 5 shows a block diagram of a sensor for use in the inventivesystem.

FIG. 6 shows a block diagram of the sensor for use in the inventivesystem of FIG. 5 wherein the absolute time clock is a GPS receiver.

FIG. 7 provides a block diagram of the sensor for use in the inventivesystem of FIG. 5 wherein electrical power to operate the sensor isinductively coupled from a power line.

FIG. 8 provides a block diagram of the sensor of FIG. 5 wherein thecommunication interface is wireless.

FIG. 9 provides a block diagram of the inventive system with threesensors in place.

FIG. 10 provides a block diagram of the inventive system configured tomonitor a large, diverse area.

FIG. 11 depicts a typical waveform representative of a gunshot.

FIG. 12 depicts the envelope of the gunshot of FIG. 11.

FIG. 13 depicts a frequency domain representation of the gunshot of FIG.11.

FIGS. 14A and 14B provide a flow chart for discriminating a particularacoustic event from other sounds received at a sensor.

FIG. 15 provides a flow chart for a preferred method for distinguishingthe type of weapon which produced a particular acoustic event.

FIG. 16 provides a flow chart for locating the source of the gunshot asreported by a sensor according to the flow chart of FIG. 12.

DETAILED DESCRIPTION

Before explaining the present invention in detail, it is important tounderstand that the invention is not limited in its application to thedetails of the construction illustrated and the steps described herein.The invention is capable of other embodiments and of being practiced orcarried out in a variety of ways. It is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and not of limitation.

Referring now to the drawings, wherein like reference numerals indicatethe same parts throughout the several views, a network of inventivesensors 22 and 30 are shown in their general environment in FIG. 1. In apreferred embodiment, a plurality of sensors 22 and 30 are dispersedover an area, typically at fairly regular intervals. Preferably, eachsensor 22 is placed such that it has a relatively unobstructed acousticview around its immediate area. By way of example and not limitation,suitable sites include: suspended from an electrical power line 122, asshown with regard to sensor 30; placed atop a building; placed atopstreetlight poles; from existing towers, in trees, and the like. In theevent gun 28 is discharged, sensors 22 and 30 receive and process theacoustic information associated with the gunshot, as discussedhereinbelow, to provide personnel with as much information as possibleabout the source of the gunfire.

As previously mentioned, sensor 22 can take on a number of embodimentswhich are adapted for placement in a particular environment. Turning toFIGS. 2 and 3, in one preferred sensor embodiment, sensor 30 isconfigured to hang from power lines. Sensor 30 comprises: a housing 34which protects its electronic circuitry from the elements; an acousticreflector 38 for directing sound waves towards a relatively protectedenvironment under housing 34; and a toroidal clip 32 which serves thedual purposes of supporting sensor 30 from a power line and inductivelycoupling electrical power from the power line into sensor 30 to providethe electrical power requirements of sensor 30.

Preferably, clip 32 is normally biased towards its closed position (asshown in FIG. 3) by a spring, or the like, (not shown). However clip 32includes lever 36 which can be used to temporarily open clip 32 forplacement over a power line 122 (FIG. 1).

Turning to FIG. 4, in another preferred embodiment, sensor 40 canalternatively be configured for mobile use, such as being clipped to asoldier or police officer. Sensor 40 includes: housing 46 which enclosesthe electronic circuitry of sensor 40; microphone 48 for receivingacoustic information; antenna 42 for communication on a wirelessnetwork; and belt clip 44 for attachment to belt, helmet, or otherarticle of clothing.

Referring to FIG. 5, preferably a sensor, such as sensor 22, includes: amicrophone 100 for receiving audible events; signal processing circuitry102 which amplifies and filters the output of microphone 100; CPU 106for performing the processing functions of sensor 22; analog to digitalconverter 104 which converts the conditioned signal from the microphone100 in to digital form for processing by CPU 106; network interface 108for communication with host 26 (FIG. 9); absolute time clock 112; andpower supply circuitry 110 for supplying the electrical needs of sensor22.

While virtually any type of microphone can be used in the inventivesensor, electret condenser or dynamic microphones are particularly wellsuited. Audible information is continuously received by microphone 100;amplified by signal conditioner 102; converted to digital information byADC 104, and analyzed by CPU 106. Generally speaking, a particular soundwill have features which distinguish that sound from other sounds.Explosive sounds, for example, are characterized by a burst of noisehaving a sharp attack, a relatively short sustain, and an exponentialdecay. For purposes of the present invention, attack, sustain, and decayare terms used to identify portions of the envelope of a sound, theenvelope being representative of the sound volume of the sound at anyparticular point in time. “Attack” refers to the beginning portion ofthe envelope, “sustain” refers to a relatively steady state condition,if one exists, following the attack, and “decay” refers to the portionof envelope terminating the sound. CPU 106 processes incoming audio toderive the envelope and optionally, spectral content to compare thesefeatures to like features of known sounds.

In addition to amplifying the signal from microphone 100, signalconditioner 102 also provides some measure of filtering of the audiosignal. Preferably, conditioner 102 includes an anti-aliasing filterwhich provides lowpass filtering at approximately one-half the samplerate of A/D converter 104. As will be appreciated by those skilled inthe art, aliasing is a well known problem associated with thedigitization of analog signals.

With further reference to FIG. 9, when CPU 106 detects a soundindicative of a particular event, the time when the event arrived at thesensor is retrieved from clock 112, logged and transmitted via interface108. Sensors 22 are connected, either by wires, or in a wirelessfashion, through a network 24 which allows communication with a hostsystem 26. If a gun 28 is fired within the audible range of one or moresensors 22, each receiving sensor detects the gunshot and transmits anidentifier and a time of arrival of the sound to the host system 26.Clock 112 is synchronized to that of every other sensor in the system sothat the relative times of arrival received from multiple sensors athost 26 are meaningful. At the host, the times of arrival from all ofthe reporting sensors are processed to determine a precise location ofthe source of the shot.

For purposes of this invention, the term “absolute time” refers to asystem for measuring time at each sensor such that the individual clock112 at any sensor operates in near perfect synchronization with theclock 112 of every other sensor. Turning then to FIG. 6, one preferredmethod for obtaining absolute time is through the use of a GPS receiver114. As is well known in the art, each GPS satellite has an internalatomic clock and precise time is periodically transmitted from eachsatellite to earth.

In addition to time of arrival information, for purposes oftriangulation, the host processor 26 must know the precise location ofeach reporting sensor. Thus, GPS receiver 114 can serve both thepurposes of determining the location of the sensor and providingabsolute time relative to the other sensors in the network.Alternatively, absolute time could be obtained through a number of othermethods such as, by way of example and not limitation, receiving WWVtime, or receiving synchronizing information from the host, or a timeserver, over network 24. It should be noted however, if such a timeclock is used, each sensor would have to be placed at a known fixedlocation such as a survey point or known landmark.

For purposes of this invention, the term “self-surveying” is used todescribe embodiments of the inventive system where the location ofsensors is determined internally. While a GPS receiver in each sensor isthe preferable means for self-surveying, other, equally well suitedmethods are known. By way of example and not limitation, the system mayself-survey by techniques such as radio distancing from a knownposition, traditional direction finding techniques between the varioussensors, etc.

Another concern for sensors placed at remote locations is obtainingelectrical power. A mobile sensor 40 is well suited for batteryoperation since the person wearing the sensor can simply recharge, orreplace, the battery at periodic intervals. Unfortunately, remotelyplaced sensors at fixed locations are probably not as easy to access forsuch purposes. For such locations a number of options are possible. Byway of example and not limitation, power supply 110 could be a batterywhich is recharged by solar cells when sunlight is available. Such asystem would require a battery of sufficient capacity to power the unitthrough periods of nighttime, snow cover, and the like.

Alternatively, power supply 110 could be a simple AC power supply wheresensor locations have conventional household electrical power available.In such installations, power supply 110 could include a battery tosurvive periods of power outage. A variation on this scheme would be toplace sensors atop streetlight poles. Power supply 110 could drawelectrical power from the street light circuit at night to charge abattery, and operate from the battery during the day when thestreetlight is off.

Turning to FIG. 7, yet another solution is that used in sensor 30. Powersupply 110 employs a current sensing coil 120 of the type typically usedto measure electrical current in AC circuits. Such coils produce avoltage proportional to the electrical current flowing through a wire122 passing through the sense coil 120. The output of the coil is thenrectified by bridge 124 and regulated to a relatively constant voltage(V+) by regulator 126. Optionally, sensor 30 can include a battery whichis recharged during periods where current is flowing through wire 122and which provides operational power for sensor 30 when insufficient orno current is flowing through wire 122. Since power lines are strungoverhead, when not buried, the lines provide both a good vantage pointfor listening and a source of electrical power which can be inductivelycoupled into sensor 30 with no physical electrical connection to thepower line. Accordingly, the voltage present on power line 122 isimmaterial.

A significant advantage of the present invention over most prior artsystems is its ability to network. Systems which perform analysis of theacoustic information at a host processor, or measure relative times ofarrival at the host, require a dedicated communication channel for eachsensor. The present invention provides the advantage that, sinceabsolute time of arrival is measured at the sensor, latency in reportingthe time to the host is immaterial to the accuracy of the calculation ofthe position. Accordingly, large numbers of sensors can share a singlecommunication channel making sensors 22 well suited for networking. Infact, network interface 108 can be of a type to access virtually anyknown networking scheme, i.e.: ethernet; token ring; the internetthrough dial-up, cellular connection, dsl, cable TV, T1, etc.; IEEE-1394schemes; USB schemes; wireless schemes; and the like. As will beapparent to those skilled in the art, wireless networking isparticularly well suited to the present invention since wirelesscommunication between sensors 22 and host 26 substantially reduces thetime of deployment and costs associated with developing supportinfrastructure.

Referring to FIG. 8, a sensor 22 is shown having a radio frequencynetwork interface 130 which communicates with external systems in awireless fashion through antenna 132. A number of wireless networksolutions are available such as wireless interfaces conforming toIEEE-802.11b or 900 MHz spread spectrum systems which are known to havea reliable range of three, or more, miles. With further reference toFIG. 9, each sensor 22 in system 20 communicates wirelessly over network24 with host 26 which also include a wireless network interface (notshown).

As a practical matter, it is envisioned that in an urban environmentthere will often be many square miles of monitored area with betweeneight and twenty sensors per square mile, depending on terrain,buildings, and other obstructions. It is likely that in such a setting,various sensor configurations will be required to adapt to particularsites. Thus, as shown in FIG. 10, a typical system will include aplurality of sensors 22 a-k. Where possible, sensors will be connectedwirelessly to the network, i.e., sensors 22 a-i, and where notpractical, connected directly to a wired network 60, i.e., sensors 22j-k.

Where host 26 is located outside of the range of the network interfaces130 (FIG. 8) of sensors 22 a-d, a separate receiver 62 may be suppliedto relay data back-and-forth between individual sensors 22 a-d and host26. Furthermore, if sensors are spread over an area larger than therange of receiver 62, additional receivers, i.e., receiver 64, may benecessary to likewise relay messages. Ideally, receivers 62 and 64connect to network 60 for communication with host 26. As will beapparent to those skilled in the art, network 60 may be either theinternet or a private network. Further, it should be noted thatreceivers 62 and 64 can also be used to communicate with other wirelessdevices such as PDAs, devices in patrol cars, etc.

Turning then to FIG. 11 wherein is shown a typical gunshot 200, takenfrom a recording of a 0.357 magnum revolver fired indoors. Analyzinggunshot 200 it can be seen that the sound exhibits a sharp attack at 202and maintains a somewhat sustained level at 204 before terminating in anexponential decay at 206. The spectral content of the noise inside theenvelope is generally random, at least over a moderate range offrequencies. Empirically, it has been found that larger caliber weaponstend to exhibit a longer decay than their smaller caliber counterparts.

With further reference to FIG. 12, an envelope 300 can be derived fromgunshot 200 which generally defines the amplitude of the sound ofgunshot 200 throughout its duration. Thus, the relative steep risingedge 302 of envelope 300 corresponds to attack 202 of gunshot 200. Thesustained level 204 and decay 206 likewise correspond to portions 304and 306 of envelope 300. Envelope 300 is generally typical of explosivetype sounds.

With still further reference to FIG. 13, the spectral content of gunshot200 can be found by performing a transformation from the time domain, asrepresented by graph 200 to the frequency domain, as represented bygraph 500, typically through a Fourier transform. As can be seen in FIG.13, the spectral content of gunshot 200 shows a noise floor 504extending from the lower end of the audible spectrum and tapering offsomewhere above 1 kilohertz. Of particular significance is the singlepredominant spike 502 at approximately 240 Hertz. Typically, significantperiodic information would be indicative of a resonance, likely from theframe of the gun, the resonance of the barrel cavity, or other likefeature of the gun. Accordingly, guns of different caliber or differentclass will exhibit strikingly different spectral content. Features whichallow identification of a weapon include: the rise time of the sound asbest indicated by portion 302; the duration of sustained noise asindicated by portion 304; the time of exponential decay as indicated by306; the width and shape of the noise floor 504; and the characteristicsof periodic energy 502.

A preferred method 600 for discriminating explosive events from othersounds is shown in the flow chart of FIGS. 14A and 14B. Starting at step600, a packet of audio samples is collected from A/D converter 104 (FIG.5). In one preferred embodiment, converter 104 is sampled atapproximately 8 kilohertz. Each packet is made up of thirty-two samplesrepresenting 4 milliseconds of information. When the packet iscollected, the absolute value is taken of each sample at step 604,essentially performing full-wave bridge rectification of the inputsignal. Next, the peak value is detected at step 606 and averaged with agroup of prior packets at 608. In the preferred embodiment, peak valueis averaged with the thirty-one immediately prior peak values to findthe next point in the envelope which is then saved at step 610.

It should be noted that envelope information can be gathered by a numberof schemes and any technique which produces envelope information issuitable for practicing the method of the present invention. However, anadvantage of the embodiment described above lies in the fact that 1024samples of data are processed using only 64 bytes of ram (assumingconverter 104 is an eight bit device). Thus the described embodiment isideally suited for use where CPU 106 is a microcontroller having limitedamounts of random access memory (hereinafter “RAM”) available. Wherespectral content is analyzed, larger amounts of RAM are required. Insuch embodiments, less memory efficient techniques may be employed toderive the envelope which provide some gain in accuracy or reduceprocessing time.

Continuing with flow chart 600, the variable “Phase” is used todetermine the amount of detection that has occurred on a sound beingreceived at step 612. “Phase” is initially zero and thus, the processfollows step 614. At step 616, if the slope of the envelope is greaterthan zero, “Phase” is incremented at step 620 indicating that the attackportion of a waveform is in process. If, at step 616, the slope of theenvelope is less than or equal to zero, the process returns to collectthe next packet. On completion of collecting the next packet, steps602-612 are performed as above. If “Phase” equals one, indicating thatthe attack portion is occurring, processing proceeds through step 622.At step 624 if the slope of the envelope is still greater than zero,processing returns to collect the next packet. If, at step 624 the slopeis less than or equal to zero, the attack portion has ended and the timeelapsed during the attack portion is measured. If the duration of theattack is of sufficient length at step 626, “Phase” is incremented toproceed to the next phase of detection. If the duration of the attack istoo long at step 626, Phase is cleared so that processing of thereceived signal will start over. As will be apparent to those skilled inthe art, the duration of the attack is significant rather than the slopeor amplitude of the attack since the amplitude will decreasesubstantially with increasing distance of the gunshot from the sensorand, with decreasing amplitude, there is a corresponding decrease inslope. Thus, the duration of the attack is a key feature in recognizinga gunshot. If the duration is less than the threshold, “Phase” isincremented to move on to the next level of detection.

Once again after collecting the next packet and building the envelopevalue in steps 602-672, processing continues through step 634 since“Phase” is equal to three. At step 636, is the envelope follows anexponential decay, an event is indicated at step 640 and absolute timeof the event is reported at step 642. As will be apparent to thoseskilled in the art, an exponential decay of the envelope will begenerally defined by the value of the envelope at the time of the end ofthe sustain portion (i.e., portion 304 of FIG. 12) and a time constant.One method to determine the time constant is to measure the length oftime which passes between the start of the decay (t0), and the time (t1)to reach a predetermined percentage of the starting value, for example,80%. An exponential decay is given by the equation:

V ₁ =V ₀ e ^(−(Vtc))

where: V1 is the voltage at time t; V0 is the initial voltage; t equalst₁−t₀; and tc is the time constant. Solving for the time constant (tc)yields:

tc=4.48(t ₁ −t ₀)

If the envelope decays in an exponential manner, the time to reach anyvoltage can be predicted. Empirically it has been determined thattesting two points displaced somewhat from t₁ (e.g., when the voltage is60% and 40% of the starting value) provides a reasonably accuratedetermination of an exponential decay.

It should be noted that the time constant is independent of receivedamplitude and further, that the time constant provides a roughestimation of the size of the weapon. In a minimal system, merelydetermining the time constant and qualifying the envelope in the mannerdiscussed above allows accurate discrimination between gunshots and mostother explosive events such as firecrackers.

More preferably however, CPU 106 is provided with sufficient RAM tostore an entire gunshot. After qualifying a sound through analysis ofthe envelope as discussed above, CPU 106 continues with spectralanalysis to identify the type of gun used. Turning then to FIG. 15,flowchart 650 provides a method for analyzing and identifying the typeof weapon by which a gunshot was produced. Starting at step 652,digitized acoustic data is constantly stored in a recirculating buffer.As the data is stored in the buffer, at step 654 envelope information isderived and processed according to the method of flowchart 600 of FIGS.14A and 14B. Once a gunshot is detected, rise time, the sustain portion,and the time constant of the decay are compared to characteristic valuesstored in a table to produce a list of candidate weapons at step 656.

Next, at step 658, the buffer is transformed to the frequency domainusing a Fourier transform. As will be apparent to those skilled in theart, there are a number of methods which may be employed to perform sucha transform. The present inventive method is not sensitive to the typeof transform used, only that a representation of the waveform in thefrequency domain is produced. Further processing of the transformed dataseparates the periodic information at 660 and the noise information at662, preferably, the noise floor is then subjected to filtering toproduce a “smoothed” curve. The separate components of the spectralinformation are then scored against stored tables of known weapons.Scoring of both the periodic information and the noise floor occur in asimilar manner. First, the event is normalized such that the largestvalue in the table is a predetermined value. Each table of known weapontypes is previously stored in a like normalized manner. Point-by-point,a score is produced by summing the absolute value of the differencebetween the values stored for the event with those of a candidateweapon. The table of the event is then shifted left one location and theprocess is repeated. Next the table is shifted right one place from theoriginal table and the process is repeated yet again. The score for acandidate weapon is then the lowest value of the three scores. At step664, the scores are then weighted to favor the periodic score over therandom score, and the weighted scores are added together for thecandidate. After the process is repeated for all candidate weapons, atstep 666, the weapon with the lowest score is identified as the mostlikely weapon used. At step 668 weapons which produce a score within apreset range, for example but not by way of limitation, 30%, are alsoreported. Preferably, along with the list of weapons, a sensor will alsoreport the scores of the reported weapons to the host.

In addition to reporting weapon types, a sensor also reports the time ofarrival of a gunshot. In the preferred embodiment, the time of arrivalis measured at the peak of the attack. While the precise feature whichis used as the reference for triggering time of arrival is somewhatarbitrary, the peak of the attack is highly recognizable.

It should be noted that some processing may also be required to obtain atime with the requisite accuracy. For example, many GPS receiversprovide a time-mark which is typically a one pulse-per-second outputsynchronized to the beginning of a GPS second within a few nanoseconds.When such a receiver is used, the timing of fractional seconds must beperformed within CPU 106 (FIG. 6), or an external device. Preferably,CPU 106 includes a timer with a capture register for capturing the countpresent at the timer upon the occurrence of an external event. Eachrising edge of the time-mark signal causes the timer to store its countin a capture register and reset the counter. Upon the occurrence of areportable event, CPU 106 retrieves the present value of the counter inCPU clock ticks. Next CPU 106 divides the value retrieved by the numberof clock cycles recorded in the capture register for the previoussecond. The significance of this step is that the accuracy of thefractional second value resulting from the divide operation is enhancedwell beyond the accuracy of the clock source, typically a crystal, tothat of the GPS clock. The accuracy of a crystal is typically specifiedto be within 0.05%. In contrast, many suppliers hold that the time-markoutput of a GPS receiver has an accuracy of one part in 10⁹.

Referring once again to FIG. 10, in a monitoring system 20 it is mostpreferable that the sensors be positioned such that at least threesensors will hear a gunshot event. When a gunshot occurs, each sensor 22a-k which receives the event performs the processing necessary torecognize the event, identify a most likely candidate weapon, anddetermine time of arrival. This information is then via network 60 tohost 26. At the host time-of-arrival information is processed totriangulate on a particular target. This process is shown in flowchart700 of FIG. 16.

At step 702, host 26 receives information from reporting sensors (e.g.,sensors 22 a-c, 22 e, and 22 k). At 704, host 26 checks to be sure threesensors have reported. It should be noted that no positional informationcan be derived from a single reporting sensor, except by assuming theevent must be very close to that sensor, and if only two sensors report,the position of the gunshot can only be predicted to lie along aparticular curve. Thus, if only one or two sensors report, the event isflagged as suspect at step 706. It should be noted that a rough positionis still supplied by host 26, the position is simply flagged as being anapproximation rather than a precise position fix.

If, as in this example, three or more sensors 22 report, host 26 selectsthe three sensors reporting the best correlation to a candidate weapon(e.g., sensors 22 b, 22 e, and 22 k). Alternatively, host 26 couldselect the three reporting sensors in closest proximity to each other,or some other quality standard. Using the locations of the threeselected sensors and the times-of-arrival at the sensors, the hostcalculates a location for the source of the gunshot through well knowntriangulation techniques as discussed in more detail hereinbelow. Next,at step 710 host 26 uses data from the remaining reporting sensors (inthis example sensors 22 a and 22 c) to determine the potential error inthe calculated position. Using the information associated with thecalculated position, host 26 tests the information from sensors 22 a and22 b. If this information results in a position different from that ofthe original calculation, host 26 calculates statistical informationwith regard to the error between the reporting sensors at step 710.Finally, at step 712, host 26 reports the position and scores the resultbased on the standard deviation calculated from the additional reportingsensors.

As mentioned above, triangulation is well known in the art and, in fact,is performed within the GPS receiver to calculate a position on earthrelative to a plurality of satellites in space. With regard to usingtimes-of-arrival and sensor locations to determine the position thesource of an event, it should first be noted that, at the time ofreporting, the precise time of the event (t₀) is unknown. Assuming anevent is detected at three or more sensors, the precise location andprecise time of the event are given by:

d ₁ =s(t ₁ −t ₀);

d ₂ =s(t ₂ −t ₀);

d ₃ =s(t ₃ −t ₀);

d ₁=((x ₁ −x ₀)²+(y ₁ −y ₀)²)^(0.5);

d ₂=((x ₂ −x ₀)²+(y ₂ −y ₀)²)^(0.5); and

d ₃=((x ₃ −x ₀)²+(y ₃ −y ₀)²)^(0.5)

where: unknown terms: x₀, y₀ are the location of the source of theevent; t₀ is the time of the event; and d_(n) the distance from theevent to sensor n. Known terms: s is the speed of sound; x_(n), y_(n)are the location of a sensor n; and t_(n) is the time of arrival atsensor n.

Thus, calculating the location and precise time of an event requires thesolving of six equations in six unknowns.

As is well known in the art, the speed of sound (s) varies somewhat withtemperature, relative humidity, and air pressure. Ideally, host 26 willeither have sensors for measuring outdoor temperature, relativehumidity, and barometric pressure, or have access to such information.Host 26 can periodically calculate the speed of sound based on localenvironmental factors, so that at the time of an event, the speed ofsound is, for all practical purposes, a constant.

As will be apparent to those skilled in the art, calculating a solutionfrom the above equations may be somewhat cumbersome, particularly inlight of the limited processing power which might be available at host26. A number of methods can be used to simplify the process of locatingthe source of an event such as assuming an initial t₀ and adjusting t₀on an iterative basis until the equations solve and then calculatingprogressively larger circles which conform to the received times fromeach sensor until a common point of intersection is found between thethree circles. Regardless of the exact technique used, triangulation toan event based on times of arrivals is well known in the art and used inareas such as seismic exploration, earthquake detection, and even GPSreceivers. Any such technique may be adapted for use with the presentinvention to obtain the requisite positional accuracy.

It should also be noted that, while not required for many environments,nonetheless, for greatest accuracy the altitude of the reporting sensorsshould be considered and the location of the shooter should be fixed inthree dimensions. If calculations are limited to two dimensions,inaccuracies can arise with events located close to a sensor since thesensor may be some distance above the ground. If, for example, thetechnique of estimating an initial time and calculating expandingcircles is used for a two dimensional fix, the technique can be extendedto a three dimensional fix by iterative calculations of expandingspheres instead.

It should also be noted that, while preferred embodiments of theinventive system rely on GPS time to synchronize the clocks at everysensor, the invention is not so limited. GPS time is presently availableand equipment exists to harness the transmitted information tosynchronize clocks worldwide with sufficient accuracy to practice thepresent invention. However, the inventive system could employ any systemfor synchronizing clocks as long as the clocks are held insynchronization with sufficient tolerance to allow satisfactorytriangulation to an event. The accuracy of the clocks affects theaccuracy of the position calculation in that, the accuracy of a positionfix is limited to the potential time difference between two clocksdivided times the speed of sound.

It should also be noted that a common theme among the various methods ofproviding electrical power is the use of a battery to operate thesystem, either as primary power, or to fill-in during periods of poweroutage. Either way, when powered from a battery it is preferable to takesteps to reduce power usage of the sensor to increase battery life. ManyGPS receivers provide a sleep mode wherein the power requirements are amere fraction of the power requirements of the operational mode.Unfortunately, time keeping functions stop in the sleep mode.

As discussed above, unlike prior art systems, latency in the delivery ofthe information related to an event does not affect the accuracy of thelocation calculation of the present invention. As a result, a sensoroperating from a battery can put its GPS to sleep until an event isdetected. Upon detection of the event, the sensor will direct the GPS towake-up and use the internal CPU clock to measure time until the GPS isfully functional and time keeping function are restored, typically a fewseconds. Once the GPS is fully functional, the accuracy of the timerecorded from the CPU clock can be enhanced using the time mark functionof the GPS as discussed in detail hereinabove. While the degree ofimprovement in battery life will vary with the type of GPS receiverselected and the number of triggering events which occur, on the averageit is believed that battery life can be extended by roughly a factor offour. In installations where a delay of eight to ten seconds inproviding the location of the source is acceptable, it is preferable touse this scheme when the unit is operating from battery power.

It should also be noted that, while the preferred embodiments have beendescribed with respect to a gunshot detector, the invention is not solimited. The invention can be adapted to detect virtually any acousticevent where the waves propagate through virtually any medium. Any suchadaptations are within the scope and spirit of the present invention.For example, the invention could be readily adapted to detect glassbreaking, screams, car crashes, etc. which propagate through air.Likewise, the invention could be readily adapted to detect and locatefootsteps, earthquakes, etc. through seismic transducers; or sonarpings, or the like, through hydraulic sensors.

It should also be noted that, where acoustic waves are constrained to alinear, or fixed, path, positional fixes can be obtained from twosensors. Such embodiments could look for events through railroad trackssuch as the cutting of a track, or detect a person climbing a fence, asat a fenced boarder, or at a prison fence, etc.

In military applications, the present invention is particularly wellsuited for sighting weapons such as mortars and artillery. In such anapplication, the system could initially detect the position of anaggressor as discussed above. Then, upon issuing return fire, the systemcan detect the point of impact of the return fire and adjust the azimuthand elevation to zero in on the target with a second shot. This methodwill correct for errors in targeting caused by wind or any othervariable in sighting. In both military and civilian applications, thepresent invention can be used to automatically direct security cameras,lighting, etc. to point to the source of a detected sound such as ascream or breaking glass.

It should be further noted that, consistent with the objects of theinvention, a system for detecting gunshots according to the presentinvention can be deployed within a few hours. Deploying sensors 30 whichparasitically take power from power lines, or sensors 22 which are solarpowered, both equipped with wireless interfaces, individual sensors canbegin reporting within seconds of installation. With a GPS in eachsensor 22 or 30, the system can self-survey as sensors are deployed.Receivers (i.e., receivers 62 and 64) can be deployed at locations whereinternet access pre-exists, or gain internet access immediately througha cellular telephone network. Each receiver can then report to anexisting application server which monitors multiple jurisdictions. Uponthe detection of a gunshot, the application server can immediatelynotify the appropriate authorities with the pertinent information aboutthe event. In fact, on a battlefield, deployment of battery operatedsensors could be as simple as dropping sensors from an airplane onto thebattlefield ahead of advancing troops.

Finally, it should also be noted that the terms employed in thediscussion of the preferred embodiments are to be given their broadestmeaning. By way of example, but not by way of limitation, the term“microprocessor” is used broadly to describe programmable devices,including but not limited to microcontrollers, risk processors, ARMprocessors, digital signal processors, logic arrays, and the like.Similarly, for purposes of this invention, the term “GPS” should bebroadly construed to include any satellite based navigation system,regardless of the country of origin.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned above as well as those inherenttherein. While presently preferred embodiments have been described forpurposes of this disclosure, numerous changes and modifications will beapparent to those skilled in the art. Such changes and modifications areencompassed within the spirit of this invention.

1. A sensor for detecting and providing a time of arrival of an acoustic event produced in the environment comprising: a microphone acoustically coupled to the environment, such that said microphone provides a signal representative of acoustic waves received at said sensor; a microprocessor, said microprocessor being in electrical communication with said microphone such that a digital representation of said signal is present in said microprocessor; an absolute time clock in digital communication with said microprocessor such that said microprocessor can obtain synchronized time from said sensor; a network interface in digital communication with said microprocessor such that said microprocessor can communicate over a computer network, wherein when a predetermined event is received at said microphone, said microprocessor obtains a time of arrival from said absolute time clock and transmits said time of arrival over said communication network.
 2. The sensor of claim 1 wherein said absolute time clock comprises a GPS receiver.
 3. The sensor of claim 2 wherein said GPS receiver also communicates a position of said sensor to said microprocessor.
 4. The sensor of claim 1 further comprising a power supply for supplying electrical power to said sensor.
 5. The sensor of claim 4 wherein said power supply comprises a solar cell and a battery.
 6. The sensor of claim 4 wherein said power supply comprises: a current sensing coil having an aperture for receiving a power line therethrough; a rectifier in communication with said current sensing coil, said rectifier providing a DC output; a voltage regulator having an input for receiving said DC output and a regulated output, wherein, when a power line is received through said aperture, electrical current flowing through the power line will induce a voltage in said current sensing coil which is rectified by said rectifier to produce said DC output.
 7. The sensor of claim 1 wherein said network interface comprises a radio frequency transceiver.
 8. The sensor of claim 1 wherein said network interface is configured to connect to an ethernet network.
 9. The sensor of claim 1 wherein said network interface is configured to communicate over a cellular telephone network.
 10. A system for locating and identifying an acoustic event comprising: a plurality of sensors for reporting a time of arrival of a known acoustic event, each of said sensors comprising: a microphone, said microphone producing a signal indicative of acoustic waves received at the sensor; a microprocessor in communication with said microphone such that information produced by said signal in response to said acoustic waves can be processed by said microprocessor to detect said known acoustic event; an absolute time clock in digital communication with said microprocessor such that said microprocessor can obtain synchronized time from said absolute time clock; and a network interface in digital communication with said microprocessor such that said microprocessor can communicate over a network; a network, each network interface of said plurality of sensors being connected to said network; and a host processor connected to said network, wherein, upon the occurrence of said known acoustic event, at least one sensor will detect said known acoustic event and, in response to said known acoustic event, report the identity of said acoustic event and the time of arrival of said known acoustic event to said host processor over said network.
 11. The system for locating and identifying an acoustic event of claim 10 wherein said plurality of sensors includes at least three sensors and said known acoustic event is received by said at least three sensors and, upon receiving said time of arrival from said at least three sensors, said host processor will calculate a location for the source of said known acoustic event.
 12. The system for locating and identifying an acoustic event of claim 11 wherein said host processor calculates a location and an initial time of said event by solving six equations in six unknown variables.
 13. A method for locating the source of an acoustic event comprising the steps of: providing at least three sensors dispersed over an area to be monitored at known locations, each sensor having a microphone for receiving an acoustic event and a synchronized clock; providing a network, each of said at least three sensors being configured for communication over said network; receiving a known acoustic event at said at least three sensors; at each of said sensors, transmitting a time of arrival of said known acoustic event over said network, said time of arrival being obtained from said synchronized clock; receiving said time of arrival from each sensor of said at least three sensors at a processor; and at said processor, triangulating the location of the source of said known acoustic event from said received times of arrival and said known locations.
 14. A method for identifying the source of a known acoustic event comprising the steps of: storing envelope and spectral characteristics of a particular acoustic event; receiving acoustic waves at a sensor; storing said acoustic waves in memory; processing the received acoustic waves to derive an envelope of the acoustic waves; performing a first correlation between points along the derived envelope and points stored envelope characteristics of said particular acoustic event; if said first correlation indicates that said derived envelope may represent an acoustic event of the type of said known acoustic event, transforming the stored acoustic wave into the frequency domain to create spectral information of said received acoustic wave; performing a second correlation of said spectral information of said stored acoustic wave to said spectral characteristics of said known acoustic event; and if a predetermined number of points of said spectral information correlate with points of said spectral characteristics, providing an identification that said acoustic waves were produced by an event matching said known acoustic event 